Response of the characteristics of organic carbon mineralization of soft rock and soil composed of sand to soil depth

Study site

The experimental plot was established in a pilot test base of the Shaanxi Provincial Land Engineering Technology Research Institute (Fuping county, China) (109°11′E, 34°42′N). The area is in the transition zone between the Guanzhong Plain and the northern Shaanxi Plateau and belongs to the gully region of the Weibei Loess Plateau. The terrain is high in the north and low in the south. It slopes from northwest to southeast. The elevation in the territory is 375.8–1, 420.7 m. The area belongs to the continental monsoon warm zone and has a semiarid climate. The annual total radiation is 5,187.4 MJ m⁻²; the annual average sunshine hours is approximately 2,389.6 h; the annual average temperature is 13.1 °C, and the annual average precipitation is 527.2 mm. The interannual precipitation varies substantially, and its coefficient of variation (CV) reaches 21.1%.

Experimental design

The test field was utilized to simulate the conditions of soft rock and sand mixed layer in the Mu Us sandy land. The design depth of the experimental field was 1 m, and it was filled with a mixture of soft rock and sand at a depth of 0–30 cm and filled with aeolian sandy soil at a depth of 30–70 cm. The test was conducted in 2009, and four treatments of soft rock and sand in a volume ratio of 0:1 (CK), 1:5 (S1), 1:2 (S2), and 1:1 (S3) were selected. Each treatment was repeated three times for a total of 12 trial plots (Fig. 1). The area of each plot was 2 m × 2 m. Based on the site conditions of the test plot, considering the uniformity of factors, such as illumination and microtopography, the test plot was arranged from south to north in a “one” shape. The experimental field was artificially sown with was maize (Jincheng 508) and wheat (Xiaoyan 22) with a rotation of two crops a year. The types of fertilizers tested in the experimental field were urea, including N 46.4%, diammonium phosphate; including N 16%, containing P₂O; 44%, potassium sulfate, including K₂O 52%, and the amount of fertilizer applied was 255 kg hm⁻² (N), 180 kg hm⁻² (P₂O₅) and 90 kg hm⁻² (K₂O). The mode of fertilization of each treatment was the same, and all three fertilizers were applied.

Collection of soil samples

After the wheat was harvested in May 2020, soil samples at a depth of 0–30 cm (0–10 cm, 10–20 cm and 20–30 cm) were collected from each plot. Soil samples were collected from five different points and mixed to make a composite sample for each plot. The animal and plant residues were removed from the collected soil samples, sieved through a 1-mm sieve, and then divided into two portions. One was placed at 4 °C for the mineralization incubation test, and one was naturally air-dried to determine the basic physical and chemical properties.

Method of determination

The soil organic carbon (SOC) was determined using the potassium dichromate-concentrated sulfuric acid external heating method as previously described (Nelson & Sommers, 1996). The total nitrogen (TN) was determined using Kjeldahl digestion; the available phosphorus (AP) was determined using molybdate blue colorimetry, and the concentrations of available potassium (AK) were measured using atomic absorption spectrometry (Qiu et al., 2014). The soil particle composition was determined using a Mastersizer 3000 laser diffraction particle size analyzer (Malvern Instruments, Worcestershire, UK) (Sun & Han, 2018).

The content of dissolved organic carbon (DOC) content was determined by extraction with deionized water as previously described (Ghani, Dexter & Perrott, 2003). The specific steps were as follows: the weight of fresh soil equivalent to that of 10 g of dry soil was placed in a 50 mL centrifuge tube, and 30 mL of deionized water was added. The mixture was shaken and extracted for 30 min at room temperature and centrifuged using a high-speed centrifuge (Allegra 64R; Beckman Coulter, New York City, NY, USA). After centrifugation, the supernatant was passed through a 0.45-µm filter membrane, and the filtrate was measured with a Multi N/C 3100 total organic carbon/total nitrogen analyzer (AnalytikJena, Jena, Germany). The soil microbial biomass carbon (SMBC) was determined using the chloroform fumigation-potassium sulfate extraction method as previously described (Brookes et al., 1985).

The organic carbon mineralization test utilized the lye absorption method (Ribeiro et al., 2010), which entailed weighing 30 g of the soil stored at 4 °C into a 50 mL beaker, adjusting the water content with deionized water to maintain the amount of water at approximately 65%, and weighing the beaker as a constant value at this time. Lye and the soil-filled beaker were placed in an incubation bottle to be sealed and cultured in the dark. The lye was a solution of 0.1 mol L⁻¹ NaOH. The lye was removed and titrated with dilute acid at 1, 3, 5, 7, 9, 16, 23, 30, 37, 44, 51, and 58 days of incubation respectively. The dilute acid was a solution of 0.1 mol L⁻¹ HCl. A volume of two mL of one mol L⁻¹ BaCl₂ solution and 2 drops of phenolphthalein indicator were added to the lye before each titration. After the dilute acid titration had been completed, a new lye solution was replaced. At this time, the beaker that contained the soil was removed and weighed, and deionized water was added to keep it at a constant weight before incubation.

Data analysis

The rate of soil organic carbon (SOC) mineralization was calculated as described by Wang et al. (2014). After the indoor incubation was completed, the first-order kinetic equation in Origin 2017 software (OriginLab, Northampton, MA, USA) was used to fit the SOC cumulative mineralization parameters to obtain the potential mineralizable organic carbon (C₀) and turnover rate constant (k) (Stanford & Smith, 1972). The formula for first-order kinetic model is shown as follows: (1)${\displaystyle {\mathrm{C}}_{\mathrm{t}}={\mathrm{C}}_0\left(1-{\mathrm{e}}^{\mathrm{-kt}}\right)}$ where C_t is the cumulative mineralization of SOC (mg kg⁻¹); C₀ is the potential mineralizable organic carbon (mg kg⁻¹), and k is the turnover rate constant (d⁻¹). The semi-turnover period was T_1∕2 = ln2/k (d).

The soil depth and compound ratio were the two factors in this study. The data from soil physical and chemical properties and mineralization were subjected to a Duncan multiple comparison and analysis of variance (ANOVA) using Microsoft Excel 2010 (Redmond, WA, USA) and SPSS 20.0 (IBM, Inc., Armonk, NY, USA).

Soil physical and chemical properties

The combined effect of the two factors of soil depth and compound ratio had a significant impact on the SOC, AK, sand, clay and texture (Table 1). The SOC, TN and clay contents of the S1, S2, and S3 treatments were significantly higher in the 0–10 cm soil depth than those of the CK treatment (P < 0.05), and there was no significant difference in the content of AP between treatments (P > 0.05). The contents of AK and silt in the S2 and S3 treatments were significantly higher than those in the CK treatment, and the content of sand was lower in the S2 and S3 treatments. There was no significant difference between the contents of SOC and AP in the 10–20 cm soil depth treatment, and the contents of TN and AK of the S3 treatment were significantly higher than those of the CK. The content of sand decreased significantly with the increase in proportion of soft rock, while the contents of silt and clay increased significantly in in the 10–20 cm soil depth treatment. During the 20–30 cm soil depth, there was no significant difference in the contents of SOC, TN, AP and AK among the treatments. The sand content of S2 treatment decreased significantly, and the clay content increased significantly. All of the nutrients were found at higher concentrations in the 0–10 cm soil depth. With the change in composition of soil particles in the compound soil, the trend of change of soil texture was sand-loamy sand-sandy loam. It was apparent that the content of soil nutrients increased significantly under the S2 and S3 treatments, with a higher content in the 0–10 cm soil depth. The soil texture developed benign.

Rate of SOC mineralization

The rate of mineralization of different types of organic carbon in the compound soils changed with the incubation time, revealing clear differences in their characteristics depending on the stage. During the early stage of incubation, the rate of SOC mineralization was unstable, and it first decreased and then increased (Fig. 2). The rate of mineralization of organic carbon of each soil depth reached its peak on day 7, and the range of variation was 56.99–78.05 mg kg⁻¹ d⁻¹ (0–10 cm), 52.64–83.13 mg kg⁻¹ d⁻¹ (10–20 cm), and 53.36–77.32 mg kg⁻¹ d⁻¹ (20–30 cm). The degree of variation could be owing to the high content of easily mineralized organic carbon in the soil, along with the decomposition of light organic carbon; a large amount of nutrients required for microbial metabolism are produced, which leads to an increase in the activity of soil microorganisms and rate of SOC mineralization (Liu & Song, 2008; Zhang et al., 2021). The rate of mineralization of SOC slowly declined and gradually stabilized during the latter period of experiment. At the end of culture, the rates of SOC mineralization of S1, S2, and S3 treatments in the 0–10 cm soil depth were 22.51 mg kg⁻¹ d⁻¹, 12.80 mg kg⁻¹ d⁻¹, and 17.83 mg kg⁻¹ d⁻¹, respectively. The rates of mineralization of the CK and S2 were basically the same, while those of S1 and S3 increased by 12.71% and 33.96%, respectively, compared with those of the CK. The average values of rate of SOC mineralization in the 10–20 cm and 20–30 cm soil depths were basically the same, but both were less than that in the 0–10 cm soil depth. In the 10–20 cm soil depth, the rate of SOC mineralization of S1, S2, and S3 treatments increased by 288.36%, 133.33%, and 351.72%, respectively, compared with that of the CK. At the 20–30 cm soil depth, the rates of mineralization of SOC of the S1, S2, and S3 treatments were lower than that of the CK treatment.

SOC cumulative mineralization (C_t)

The C_t in each soil depth increased exponentially with the incubation time, and the average value gradually decreased as the soil depth deepened. The specific performance was 0–10 cm (843.72 mg kg⁻¹) > 10–20 cm (659.16 mg kg⁻¹) > 20–30 cm (612.30 mg kg⁻¹) (Fig. 3). In the same soil depth, the C_t differed between treatments with different compound ratios. In the 0–10 cm soil depth, the S1 treatment had the largest C_t, which was 1,125.17 mg kg⁻¹, followed by the CK, S3, and S2 treatments. In the 10–20 cm soil depth, the S3 treatment had the largest C_t, which was 781.91 mg kg⁻¹, followed by those of the S1, S2, and CK treatments. In the 20–30 cm soil depth, the CK treatment had the largest C_t, which was 796.49 mg kg⁻¹, followed by the S3, S2, and S1 treatments. When all the soil depths were factored into account, the average cumulative mineralization of organic carbon was the largest in S1 treatment, which was 806.62 mg kg⁻¹, followed by the S3, CK and S2 treatments, and the cumulative mineralization of organic carbon was 721.30 mg kg⁻¹, 697.14 mg kg⁻¹, and 595.19 mg kg⁻¹, respectively.

Cumulative rate of SOC mineralization

The accumulated rate of mineralization (C_r) of the SOC in compound soils of different soil depths differed to some extent. The C_r in the 0–10 cm (26.01%) soil depth was significantly higher than that at the 10–20 cm (19.08%) and 20–30 cm (19.52%) soil depths (Fig. 4). In the 0–10 cm soil depth, the ratios of S1, S2, and S3 were reduced by 1.61-, 2.49-, and 2.44-fold compared with the CK, respectively. Compared with the S1 treatment, the C_r of SOC under S2 and S3 treatments was significantly reduced by 1.55- and 1.52-fold. In the 10–20 cm soil depth, the C_r of SOC treated by S1, S2 and S3 did not differ significantly but increased to some extent compared with that treated by the CK. In the 20–30 cm soil depth, there was no significant difference in the C_r of SOC treated by S1, S2, and S3, but it was significantly reduced by 1.17−1.29-fold compared with that treated by the CK. It was apparent that the S2 and S3 treatments had a lower C_r, which is beneficial to carbon storage, and the effect at the 10–20 cm soil depth was even better.

SOC mineralization kinetic equation fitting

Based on the CO₂-C release measured at each stage of incubation in the 58 d constant room temperature, the C_t in soil depth with different compound ratios of 0–10 cm, 10–20 cm and 20–30 cm was fitted using a first-order kinetic equation (C_t = C₀[1–e-^kt]). The results showed that the fitting effect was excellent (R² > 0.96), and fitting parameters, such as C₀, k and T_1∕2, were obtained (Table 2). Among all the soil depths, the 0–10 cm soil depth has the largest C₀. The C₀ and C_t of 0–10 cm and 10–20 cm soil depths have the same tendency to change, and the C₀ value of the 20–30 cm soil depth was the largest when treated with S1 and S3. The k value was the largest in the 10–20 cm soil depth, and there was no significant difference between the 0–10 cm and 20–30 cm soil depths. The k value of the S1 treatment in the 0–10 cm and 20–30 cm soil depths was significantly lower than those of the S2 and S3 treatments. The tendency of T_1∕2 to change was opposite to the value of k, and the T_1∕2 of 0–10 cm and 20–30 cm soil depths were larger. The C₀/SOC in the 10–20 cm soil depth was the smallest (25.47%), and the C₀/SOC in the S2 and S3 treatments was small (21.48% and 28.27%, respectively). In summary, the mineralization of the 10–20 cm soil depth was relatively small, and the ability of the S2 and S3 treatments to sequester carbon was relatively strong.

Characteristics of the variation of DOC and SMBC during SOC mineralization

Before and after the incubation, the average DOC content decreased with the deepening of the soil depth and increased with the increase in the proportion of soft rock. The average content of SMBC was the largest in the 10–20 cm soil depth before and after incubation, and the increase paralleled that of the proportion of soft rock. With the extension of the incubation time, the contents of DOC and SMBC tended to decrease compared with the initial content. After the incubation, the contents of DOC in the 0–10 cm, 10–20 cm, and 20–30 cm soil depth decreased by 17.76–23.21%, 19.86–24.46%, and 14.99–20.58%, respectively, compared with those before incubation. The contents of SMBC were reduced by 14.55–21.54%, 12.69–20.01% and 19.34–20.99%, respectively, compared with before incubation (Table 3). In the 0–10 cm soil depth, there was no significant difference in the content of DOC between the treatments before and after the incubation, and the content of SMBC was the largest in the S3 treatment. In the 10–20 cm soil depth, the contents of DOC of the S1, S2, and S3 treatments before and after the incubation were significantly higher than that of the CK treatment, and the content of SMBC was the highest in S2 treatment. In the 20–30 cm soil depth, the changes of DOC and SMBC before and after the incubation were the same, and the contents of both S2 and S3 treatments were higher. It is apparent that the contents of DOC and SMBC of the S2 and S3 treatments have a greater tendency to change, and the mineralization was stronger, particularly in the 10–20 cm soil depth.

Relationship between the parameters of SOC mineralization and physicochemical properties

Through the correlation analysis of the C₀ and k values of three soil depths and four soils with different compound ratios and soil physicochemical properties, the results showed that there was no significant correlation between the C₀ and k values at the 0–10 cm soil depth and the soil physicochemical properties (Table 4). As the soil depth deepened, the correlation became clearer. At the 10–20 cm soil depth, the C₀ value had no significant correlation with the soil physicochemical properties, but the k value had a significant negative correlation with the contents of SOC and silt and an extremely significant negative correlation with the AK content. In the 20–30 cm soil depth, the C₀ value significantly positively correlated with the AK content. The k value significantly positively correlated with the contents of TN and DOC and was extremely significantly positively correlated with the content of SMBC. It showed that the influence of soil depth and compound ratio on C₀ and K of the 0–10 cm soil depth was not significant and had little effect on mineralization, while the influence of 10–20 cm and 20–30 cm soil depths on the mineralization of organic carbon was higher.